Laser apparatus and reservoir computing system

ABSTRACT

To realize a reservoir computing system with a small size and reduced learning cost, provided is a laser apparatus including a laser; a feedback waveguide that is operable to feed light output from the laser back to the laser; an optical splitter that is provided in a path of the feedback waveguide and is operable to output a portion of light propagated in the feedback waveguide to outside; and a first ring resonator that is operable to be optically connected to the feedback waveguide, as well as a reservoir computing system including this laser apparatus.

BACKGROUND Technical Field

The present invention relates to a laser apparatus and a reservoircomputing system.

Related Art

A reservoir computing system that uses a recurrent network structurereferred to as a reservoir is known as a learning method for handlingtime-series data, such as for voice recognition and securitiespredictions. Using a laser apparatus as the reservoir of such areservoir computing system is also known.

Such a reservoir computing system preferably uses the reservoir topropagate a signal component therein in a complex manner, in order tolearn a complex input/output characteristic. It is known that the laserapparatus can realize a complex nonlinear input/output characteristic byusing external feedback light obtained by lengthening the externalpropagation distance of the light. However, when the externalpropagation distance of the external feedback light of the laserapparatus is lengthened, the size of the laser apparatus is increased,and therefore it is difficult to realize a small-scale reservoircomputing system.

SUMMARY

According to a first aspect of the present invention, provided is alaser apparatus comprising a laser; a feedback waveguide that isoperable to feed light output from the laser back to the laser; anoptical splitter that is provided in a path of the feedback waveguideand is operable to output a portion of light propagated in the feedbackwaveguide to outside; and a first ring resonator that is operable to beoptically connected to the feedback waveguide, as well as a reservoircomputing system comprising this laser apparatus.

The summary clause does not necessarily describe all necessary featuresof the embodiments of the present invention. The present invention mayalso be a sub-combination of the features described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary configuration of a reservoir computing system100.

FIG. 2 shows an exemplary configuration of a reservoir computing system200 according to the present embodiment.

FIG. 3 shows an exemplary operational flow of the reservoir computingsystem 200 according to the present embodiment.

FIG. 4 shows an exemplary delay coordinate vector generated by thevector generator section 220 according to the present embodiment.

FIG. 5 shows exemplary learning results of the reservoir computingsystem 200 according to the present embodiment described above.

FIG. 6 shows an example of a spike neuron model used by the reservoircomputing system 200 according to the present embodiment.

FIG. 7 shows a modification of the reservoir computing system 200according to the present embodiment.

FIG. 8 shows a first exemplary configuration of the reservoir 130according to the present embodiment.

FIG. 9 shows a second exemplary configuration of the reservoir 130according to the present embodiment.

FIG. 10 shows an exemplary configuration of the laser apparatus 500according to the present embodiment.

FIG. 11 shows an exemplary configuration of a cross section of the laserapparatus 500 shown in FIG. 10 across the line A-B.

FIG. 12 shows an example of the light output characteristic of a laserapparatus serving as a comparison target.

FIG. 13 shows an example of the light output characteristic of the laserapparatus 500 according to the present embodiment.

FIG. 14 shows an exemplary hardware configuration of a computeraccording to the embodiment of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, some embodiments of the present invention will bedescribed. The embodiments do not limit the invention according to theclaims, and all the combinations of the features described in theembodiments are not necessarily essential to means provided by aspectsof the invention.

FIG. 1 shows an exemplary configuration of a reservoir computing system100. The reservoir computing system 100 may be an apparatus thatperforms learning based on input data, output data, and training data.Furthermore, the reservoir computing system 100 may be operable toperform testing and make predictions for output data corresponding tothe input data, based on the learning results. The reservoir computingsystem 100 adjusts weights in the system, in a manner to reduce thedifference between output data that is output in response to the inputdata and training data corresponding to the input data. The reservoircomputing system 100 includes a data generator 110, an input layer 120,a reservoir 130, an output layer 140, an adaptive filter 150, acomparing section 160, and a learning processing section 170.

The data generator 110 may be operable to generate the input data andsupply the reservoir computing system 100 with the input data. If thereservoir computing system 100 is performing learning, the datagenerator 110 may generate training input data and training datacorresponding to this input data, and supply the reservoir computingsystem 100 with this input data and training data. Furthermore, when thereservoir computing system 100 performs a test, makes a prediction, orthe like based on learning results, the data generator 110 may generateinput data for testing and supply the reservoir computing system 100with this input data.

The data generator 110 is connected to an external database 1000 or thelike, and may be operable to acquire the input data and the trainingdata. Instead, the data generator 110 may generate the input data andthe training data, using a predetermined algorithm or the like. Theinput data is a time-series data sequence in which a plurality of piecesof data are arranged according to the time axis, such as audio data,video data, or the like, for example. Furthermore, the training data maybe expected output data that is expected for the input data.

The data generator 110 may read and acquire input data stored in apredetermined format. Furthermore, the data generator 110 may beconnected to a network or the like and acquire input data and the likevia this network. Instead of or in addition to this, the data generator110 may be connected to an apparatus manipulated by a user, an apparatus(sensor) that detects and outputs time-series data, or the like, andacquire the time-series input data. The data generator 110 may store theacquired input data and the like in a storage apparatus or the likeinside the reservoir computing system 100.

The input layer 120 may be operable to input the input data from thedata generator 110. The input layer 120 may include one or more inputnodes 10. The input data may be input to one or more corresponding inputnodes 10. Each input node 10 may be operable to supply the reservoir 130with an input signal corresponding to the input data. Each input node 10may be operable to supply the input signal to one or more correspondingnodes of the reservoir 130. A weight may be set between each input node10 and one or more corresponding nodes of the reservoir 130. Each inputnode 10 may supply an input signal that has been multiplied by theweight set for the input data to the one or more corresponding nodes.

The reservoir 130 may be operable to output an inherent output signal inresponse to the input signal. The reservoir 130 may have a networkincluding a plurality of nodes. The reservoir 130 may have a recurrenttype network structure. Each of the plurality of nodes of the reservoir130 may be a nonlinear node that issues a nonlinear response to theinput signal. The plurality of nodes of the reservoir 130 may be virtualnodes. In the network including the reservoir 130, each of the pluralityof nodes may supply other corresponding nodes with a response signalcorresponding to the input signal. In this case, each of the pluralityof nodes may supply the other corresponding nodes with a weightedresponse signal.

The output layer 140 may be operable to output the response signaloutput by each node of the reservoir 130. The output layer 140 mayinclude a plurality of output nodes 20. The output layer 140 preferablyincludes a number of output nodes 20 that is substantially the same asthe number of nodes of the reservoir 130. For example, the plurality ofoutput nodes 20 correspond one-to-one with the plurality of nodes of thereservoir 130. Each output node 20 may be operable to output an outputvalue corresponding to the output signal output by the reservoir 130 inresponse to the input data. Each output node 20 may be operable tooutput to the adaptive filter 150 an output value of a correspondingnode of the reservoir 130.

The adaptive filter 150 may be operable to output the output data basedon the output value output from each output node 20. For example, theadaptive filter 150 applies weights corresponding respectively to theoutput value output by each output node 20, and outputs the resultobtained by performing a predetermined calculation as the output data.As an example, the adaptive filter 150 outputs, as the output data, thesum of each of the weighted values of the output values of the outputnodes 20. If the number of output nodes 20 is n, for example, theadaptive filter 150 outputs, as the output data, the sum Σw_(n)V_(n) ofthe values obtained by multiplying the n output values V_(n) by thecorresponding n weights (w₁, w₂, . . . , w_(n)). The adaptive filter 150may be operable to supply the comparing section 160 with the outputdata.

The comparing section 160 may be operable to compare the expected outputdata output by the data generator 110 to the output data output by theadaptive filter 150. For example, the comparing section 160 outputs thedifference between the expected output data and the output data as anerror. The comparing section 160 may supply the learning processingsection 170 with this error as the comparison result.

Furthermore, the comparing section 160 may be operable to perform acomparison operation if the reservoir computing system 100 is learning.The comparing section 160 may be operable to, if the reservoir computingsystem 100 is performing a test or making a prediction using learningresults, output the output data of the adaptive filter 150 to theoutside as-is. In this case, the comparing section 160 may be operableto output the output data to an external output apparatus such as adisplay, a storage apparatus, and an external apparatus such as thedatabase 1000.

The learning processing section 170 may be operable to set the pluralityof weights of the adaptive filter 150 according to the comparisonresults of the comparing section 160. The learning processing section170 may be operable to set the plurality of weights such that thereservoir computing system 100 outputs the output data that is expectedin response to the input data. The learning processing section 170 maybe operable to update the plurality of weights in a manner to furtherreduce the error between the output data output by the adaptive filter150 in response to the training input data being supplied to the inputnode 10 and the expected output data that is expected for the traininginput data. The learning processing section 170 may be operable tooperate if the reservoir computing system 100 is learning.

The reservoir computing system 100 described above may be a systemcapable of learning by updating the weights of the adaptive filter 150.Specifically, the reservoir computing system 100 may be operable toperform learning by updating the weights of the adaptive filter 150while the weights between the input layer 120 and the reservoir 130 andthe weights within the reservoir 130 are fixed at randomly determinedinitial values, for example.

Furthermore, by fixing the weights of the adaptive filter 150 at thelearned weights and inputting input data for testing, the reservoircomputing system 100 can output test results or prediction results forthe input data for testing. Such a reservoir computing system 100 cansimulate a learning operation and a testing operation by performingmatrix calculations. Furthermore, if the reservoir computing system 100is a physical device that outputs a nonlinear output signal in responseto an input signal, the reservoir computing system 100 can be used as areservoir 130, and is therefore expected to be a system with easyhardware installation.

However, if such a reservoir computing system 100 is actuallyimplemented as physical systems, the output signals must be extractedfrom the plurality of nodes of the reservoir 130. However, it isdifficult to extract the output signals from all of the nodes of theinternal network of the reservoir 130. Furthermore, if the reservoir 130uses virtual nodes, it is difficult to extract the output signals fromthe virtual nodes. Yet further, if a failure occurs in the attempt toextract the output signals from a portion of the nodes of the reservoir130, even if the reservoir 130 operates correctly it is impossible toaccurately perform the learning, testing, and the like if this failureis not resolved.

Therefore, the reservoir computing system according to the presentembodiment performs the learning, predicting, and the like based on theoutput signals of a portion of the nodes of the reservoir 130. Thefollowing describes such a reservoir computing system.

FIG. 2 shows an exemplary configuration of a reservoir computing system200 according to the present embodiment. In the reservoir computingsystem 200 according to the present embodiment, components havingsubstantially the same operation as components of the reservoircomputing system 100 shown in FIG. 1 are given the same referencenumerals and descriptions thereof are omitted. The reservoir computingsystem 200 according to the present embodiment includes the datagenerator 110, the input layer 120, the reservoir 130, an output layer210, a vector generator section 220, an adaptive filter 230, and alearning processing section 240.

The output layer 210 may be operable to output response signals that areoutput by a portion of the plurality of nodes of the reservoir 130. Theoutput layer 210 may include one or more output nodes 22. The outputlayer 210 may include a number of output nodes 22 that is less than thenumber of nodes of the reservoir 130. In other words, only some of thenodes among the plurality of nodes of the reservoir 130 are connected tooutput nodes 22.

FIG. 2 shows an example in which the output layer 210 includes oneoutput node 22, and only one node among the plurality of nodes of thereservoir 130 is connected to the output node 22. The output node 22 maybe operable to output an output value corresponding to an output signaloutput by the reservoir 130 in response to the input data. The outputnode 22 may be operable to output the output value of the correspondingnode of the reservoir 130 to the vector generator section 220.

The vector generator section 220 may be operable to generate amultidimensional vector based on the output value output from the oneoutput node 22 and a plurality of timings. The vector generator section220 may be operable to generate a d-dimensional delay coordinate vectorbased on one-dimensional time-series data and d timings. In this case,the vector generator section 220 may generate d data sequences.

Here, if the time-series data is a continuous value, the time differencebetween temporally adjacent timings among the plurality of timings is aninterval T. Specifically, if the time-series data is a continuous value,the vector generator section 220 may generate the d-dimensional delaycoordinate vector with d timings at intervals T. If the time-series datais a digital value, the vector generator section 220 may generate thed-dimensional delay coordinate vector with d timings corresponding tothe clock period.

If n is the degree of freedom of the reservoir 130, i.e. the number ofnodes in the reservoir 130, the vector generator section 220 maygenerate a d-dimensional delay coordinate vector in which d is greaterthan n. In this case, the vector generator section 220 preferablegenerates a d-dimensional delay coordinate vector in which d is greaterthan 2n. If the output layer 210 includes a plurality of output nodes22, the vector generator section 220 may generate a delay coordinatevector for each output node 22. The vector generator section 220supplies the adaptive filter 230 with the generated delay coordinatevectors.

The adaptive filter 230 may be operable to output output data based onresults obtained by weighting a plurality of output values output fromthe output node 22 at a plurality of timings with a plurality ofweights. The adaptive filter 230 may use a plurality of weightscorresponding to the dimensions d of the delay coordinate vector. Forexample, for each of the d data sequences, the adaptive filter 230 mayweight this data sequence using d weights.

The adaptive filter 230 may generate and output the output data from theweighted d-dimensional delay coordinate vector. The adaptive filter 230may be operable to supply the comparing section 160 with the outputdata. If the reservoir computing system 200 is learning, the comparingsection 160 may supply the learning processing section 240 with thedifference between the expected output data and the output data as theerror. Furthermore, the comparing section 160 may be operable to, if thereservoir computing system 100 is performing a test or making aprediction using the learning results, output the output data of theadaptive filter 150 to the outside as-is.

The learning processing section 240 may be operable to set the pluralityof weights of the adaptive filter 150 according to the comparisonresults of the comparing section 160. The learning processing section240 may be operable to set the plurality of weights such that the outputdata expected for the input data is output by the reservoir computingsystem 200. The learning processing section 240 may be operable toupdate the plurality of weights in a manner to reduce the error betweenthe output data output by the adaptive filter 230 in response to thetraining input data being supplied to the input nodes 10 and theexpected output data that is expected for the training input data. Thelearning processing section 240 may be operable to operate if thereservoir computing system 100 is performing learning.

Furthermore, the learning processing section 240 may be operable toadjust at least a portion of the parameters for generation of the delaycoordinate vector by the adaptive filter 230. The learning processingsection 240 may be operable to, if the time-series data is a continuousvalue, adjust one or both of the number of dimensions d and the intervalT. Furthermore, the learning processing section 240 may be operable to,if the time-series data is a digital value, adjust the number ofdimensions d.

In the manner described above, the reservoir computing system 200 may bea system capable of learning based on the output values and the expectedoutput data of a portion of the nodes among the plurality of nodes ofthe reservoir 130. The following describes the operation of such areservoir computing system 200.

FIG. 3 shows an exemplary operational flow of the reservoir computingsystem 200 according to the present embodiment. In the presentembodiment, the reservoir computing system 200 may be operable toperform learning by performing the processes from S310 to S370.

First, at S310, the weights of the reservoir 130 may be initially set.The reservoir computing system 200 may perform initial setting of theweights between the input layer 120 and the reservoir 130 and of theweights inside the reservoir 130. The weights between the input layer120 and the reservoir 130 and of the weights inside the reservoir 130may be determined using random numbers. The weights between the inputlayer 120 and the reservoir 130 and of the weights inside the reservoir130 do not need to change according to learning performed after beingdetermined once.

Next, at S320, the input data may be supplied to the input layer 120.The data generator 110 may supply the input layer 120 with the traininginput data generated by the data generator 110. Here, the data generator110 may generate the expected output data corresponding to the traininginput data supplied to the input layer 120 and supply this expectedoutput data to the comparing section 160.

Next, at S330, the output layer 210 may acquire the output signalsoutput by a portion of the nodes of the reservoir 130. In the presentembodiment, an example is described in which the output layer 210includes one output node 22 and acquires the output signal of one nodeamong the plurality of nodes of the reservoir 130.

Next, at S340 the vector generator section 220 may generate thed-dimensional delay coordinate vector. Here, if the time-series data isa continuous value, the vector generator section 220 may generate thedelay coordinate vector using d timings and the parameter of theinterval T between the timings, as shown in the following expression.Here, x(t) is one-dimensional time-series data output from the oneoutput node 22.

x(t),t∈

→{x(t),x(t−T), . . . ,x(t−(d−1)T)}∈

^(d)  Expression 1:

Furthermore, if the time-series data is a digital value, the vectorgenerator section 220 may generate the delay coordinate vector using dtimings, as shown in the following expression.

x(n),n∈

→{x(n),x(n−1), . . . ,x(n−(d−1))}∈

^(d)  Expression 2:

Next, at S350, the adaptive filter 230 may generate and output theoutput data by applying the weights to the delay coordinate vector. Forexample, the adaptive filter 230 may multiply the d data sequencesrespectively by d corresponding weights (w_(t1), w_(t2), . . . w_(td)).Furthermore, the adaptive filter 230 may output, as the output data, theweighted delay coordinate vector configured as one-dimensionaltime-series data. Specifically, the adaptive filter 230 may calculatethe dot product of the delay coordinate vector and a weight vector(w_(t1), w_(t2), . . . , w_(td)) having d weights as elements.

Next, at S360, the learning processing section 240 may update theweights of the adaptive filter 230. The learning processing section 240may update the d weights in a manner to reduce the error between theexpected output data and the output data. The learning processingsection 240 may update the weights of the adaptive filter 230 such thatthe output data becomes closer to the expected output data, using theleast squares method. In this case, the learning processing section 240may update the d weights using a linear filter.

Furthermore, the learning processing section 240 may update the weightsof the adaptive filter 230 in a manner to minimize the square error. Inthis case, the learning processing section 240 may update the d weightsusing a Wiener filter.

If the learning is to continue (S370: Yes), the reservoir computingsystem 200 may return to step S320 and perform the next learning usingthe next training input data and expected output data. The reservoircomputing system 200 may repeat the update of the weights of theadaptive filter 230 a predetermined number of times to determine theseweights. If the difference in the value before the weight update andafter the weight update is greater than or equal to a predeterminedthreshold value even after the update of the weights of the adaptivefilter 230 has been performed the predetermined number of times, thelearning processing section 240 may stop the learning and notify theuser that the weights do not converge.

If the learning is to end (S370: No), the reservoir computing system 200may determine the weights of the adaptive filter 230 to be the mostrecently updated weights. The reservoir computing system 200 may storethe determined weights in an internal storage section and/or an externaldatabase 1000 or the like. In the manner described above, the reservoircomputing system 200 may complete the learning operation.

By applying the determined weights to the adaptive filter 230 andinputting testing input data, the reservoir computing system 200 canoutput test results or prediction results for this testing input data.The following describes the input data and the delay coordinate vectorof such a reservoir computing system 200.

FIG. 4 shows an exemplary delay coordinate vector generated by thevector generator section 220 according to the present embodiment. FIG. 4shows an example in a case where the input data is a continuous value.In FIG. 4, the horizontal axis indicates time t and the vertical axisindicates the signal strength V. The curve v(t) shows an example of theinput data.

The vector generator section 220 may set a data sequence X₁ having asignal strength v(t_(m)) at the time t_(m) at every interval T from thesignal strength v(t₁) at the timing t₁ as the first vector element ofthe delay coordinate vector. Furthermore, the vector generator section220 may set a data sequence X₂ having a signal strength v(t_(m)) at thetime t_(m) at every interval T from the signal strength v(t₂) at thetiming t₂ as the second vector element of the delay coordinate vector.In the same manner, the vector generator section 220 may extract datasequences up to the data sequence X_(d) of the d vector elements and setthese data sequences from the data sequence X₁ to the data sequenceX_(d) as the delay coordinate vector.

In this case, the adaptive filter 230 may calculate the data sequencew_(t1)X₁ by multiplying each piece of data in the data sequence X₁ bythe weight w_(t1). In the same manner, the adaptive filter 230 maycalculate the data sequences w_(t1)X₁, w_(t2)X₂, w_(t3)X₃ by multiplyingeach piece of data in each of the data sequences from the data sequenceX₂ to the data sequence X_(d) by the corresponding weights from w_(t2)to w_(td).

The adaptive filter 230 may then calculate the data sequence w_(t1)X₁ atthe timing t₁ and calculate the data sequence w_(t2)X₂ at the timing t₂.Here, among the pieces of data of the data sequence w_(t1)X₁ and thedata sequence w_(t2)X₂, pieces of data with the same timings may beadded together. In the same manner, the adaptive filter 230 maycalculate each data sequence from the data sequence w_(t3)X₃ to the datasequence w_(td)X_(d) corresponding to the timings from the timing t₃ tothe timing t_(d), and output the generated time-series data as theoutput data.

FIG. 4 shows an example in which the input data is a continuous value,but instead the input data may be a digital signal expressed by a signalstrength v(t₁+(m−1)T_(c)) with a constant clock period T_(c). In thiscase, in the same manner as in the example where the input data is acontinuous value, the vector generator section 220 may set the datasequence X₁ from the signal strength v(t₁) at the timing t₁ as the firstvector element of the delay coordinate vector. Furthermore, the vectorgenerator section 220 may set the data sequence X₂ from the timingt₂=t₁+T_(c) as the second vector element of the delay coordinate vector.

In the same manner, the vector generator section 220 may extract thedata sequences up to the data sequence X_(d) of the d-th vector elementand set the data sequences from the data sequence X₁ to the datasequence X_(d) as the delay coordinate vector. In this case, theinterval T may be substantially equal to the clock period T_(c). In thisway, regardless of whether the input data is a continuous value or adigital value, the adaptive filter 230 can output the output data usingthe same operation.

An “Embedding Theorem” according to Takens is known as a technique forinvestigating a dynamics model, if the structure of the dynamics modelis unknown, by performing a reconfiguration using the time delaycoordinates corresponding to the time-series data that has actually beenmeasured. The reservoir computing system 200 uses such an embeddingtheorem, and therefore can perform the learning and testing operationscorresponding to the state of the reservoir 130 without using the outputvalues of all of the nodes of the reservoir 130.

FIG. 5 shows exemplary learning results of the reservoir computingsystem 200 according to the present embodiment described above. FIG. 5shows the steps by which the reservoir computing system 200 furtheradjusts the parameter d used by the vector generator section 220. InFIG. 5, the horizontal axis indicates the dimensions d of the delaycoordinate vector. In FIG. 5, the vertical axis indicates the evaluationvalue of the error. The evaluation value of the error is the normalizedroot mean square error, and is abbreviated as NRMSE. The NRMSE isexpressed as shown in the following expression.

$\begin{matrix}{{NRMSE} = \sqrt{\frac{\langle\left( {y - y^{\prime}} \right)^{2}\rangle}{\langle\left( {y - {\langle y\rangle}} \right)^{2}\rangle}}} & {{Expression}\mspace{14mu} 3}\end{matrix}$

In Expression 3, the expected output data, which is the training data,is y, and the output data is y′. Specifically, the denominator inExpression 3 indicates the standard deviation of the expected outputdata y, and the numerator indicates the expected value of the square ofthe error y-y′. The evaluation value NRMSE indicates that the learningis effective if the value is smaller than 1 and indicates that theeffect of the learning is higher when this value is closer to 0.Specifically, the evaluation value NRMSE is an evaluation value thatapproaches 0 as the error y-y′ becomes smaller.

FIG. 5 shows an example of results obtained by the reservoir computingsystem 200 learning an Echo Stat Network as shown by the followingexpression.

$\begin{matrix}{\mspace{79mu} {{{x\left( {n + 1} \right)} = {\tanh \left( {{W_{res}{x(n)}} + {W_{i\; n}{u(n)}}} \right)}}\mspace{79mu} {{y^{\prime}(n)} = {{W\_ out}\; {x(n)}}}{{y(n)} = {{0.3{y\left( {n - 1} \right)}} + {0.05\; {y\left( {n - 1} \right)}{\sum\limits_{i = 1}^{10}{y\left( {n - i} \right)}}} + {1.5\; {u\left( {n - 1} \right)}{u\left( {n - 10} \right)}} + 0.1}}}} & {{Expression}\mspace{14mu} 4}\end{matrix}$

Here, u(n) represents the input data input to the input node 10, x(n)represents reservoir state vector corresponding to the input data, andy′ (n) represents the output data. Furthermore, W_(in) represents thecoupled matrix between the input node 10 and the reservoir 130, W_(res)represents the coupled matrix inside the reservoir 130, and W_(out)represents the coupled matrix between the reservoir 130 and the outputnode 22. Yet further, W_(res) may be a sparse matrix with a connectivityof approximately 0.3, for example.

In addition, y(n) represents the expected output data. The expectedoutput data y(n) in Expression 4 is a model that is known as a NARMA(10) model used as a benchmark in reservoir computing. Here, NARMA is anabbreviation for Nonlinear Auto Recursive Moving Average.

FIG. 5 shows an example of results obtained by the reservoir computingsystem 200 learning the NARMA (10) model for every dimension d of thedelay coordinate vector using the reservoir 130 including 100 nodestherein. The circle marks plotted in FIG. 5 indicate learning results ofthe reservoir computing system 200. Furthermore, the triangular marksplotted in FIG. 5 indicate results of a test using the learning results.

FIG. 5 uses a dotted line to show the learning results of the reservoircomputing system 100 using the output data from all of the output nodes20, as described in FIG. 1, for comparison. The single-dash lineindicates the test results obtained using the learning results of thereservoir computing system 100. Since the reservoir computing system 100using all of the output nodes 20 does not use a delay coordinate vector,the learning results and test results are substantially constant valuesthat are unrelated to the value d of the horizontal axis.

From FIG. 5, it is understood that if the number of dimensions is lessthan or equal to 100, which is the number of nodes in the reservoir 130,the performance enters “under-fitting” territory where the reservoircomputing system 200 does not have an effect despite performing thelearning. It is also understood that if the number of dimensions d ofthe delay coordinate vector exceeds approximately twice the number ofnodes (100) of the reservoir 130, the reservoir computing system 200 canperform learning with high accuracy. Yet further, it is understood thatif the number of dimensions d of the delay coordinate vector exceedsapproximately 500, the reservoir computing system 200 enters into“over-fitting” territory.

Accordingly, the learning processing section 240 may be operable toadjust the number of timings among the plurality of timings, i.e. thenumber of dimensions d, in a manner to further decrease the error. Thelearning processing section 240 may be operable to compare the learningresults corresponding to the plurality of dimensions d and set asuitable number of dimensions d, as shown in the example of FIG. 5.Furthermore, the learning processing section 240 may be operable to alsoadjust the interval T between the plurality of timings. The learningprocessing section 240 may be operable to compare the learning resultsfor a plurality of intervals T and set a suitable interval T, as shownin the example of FIG. 5. The learning processing section 240 may beoperable to adjust at least one of the number of dimensions d and theinterval T.

The learning processing section 240 may be operable to adjust at leastone of the number of timings among the plurality of timings and theinterval T between the plurality of timings, using cross-validation.Specifically, the learning processing section 240 may perform learningusing a portion of a plurality of groups of input data and expectedoutput data corresponding to this input data. The learning processingsection 240 may perform testing of the remaining groups using thelearning results, and calculate the evaluation value NRMSE according tothe error between the output data and the expected output data. FIG. 5shows an example of evaluation results obtained by the learningprocessing section 240 using such cross-validation.

In the manner described above, the reservoir computing system 200according to the present embodiment can perform learning by using theoutput of a portion of the plurality of nodes of the reservoir 130 togenerate a delay coordinate vector from output values at a plurality oftimings. If many types of input/output data are to be learnedaccurately, there is an idea to increase the number of nodes in thereservoir 130 and create a more complex system. In this case, it becomeseven more difficult to reliably connect to all of the output nodes andextract the output data.

However, the reservoir computing system 200 according to the presentembodiment can restrict the increase in the number of connections tooutput nodes and perform learning by increasing the number of dimensionsd, and can therefore easily adapt to a complex reservoir 130.Furthermore, since learning is possible with just a small amount of anincrease in the number of dimensions d of the reservoir computing system200, e.g. from the total number of nodes to twice the total number ofnodes, learning can be performed without significantly increasing theamount of calculations.

The reservoir computing system 200 according to the present embodimentdescribed above may use a spike neural network or the like as thereservoir 130. FIG. 6 shows an example of a spike neuron model used bythe reservoir computing system 200 according to the present embodiment.FIG. 6 shows neuromorphic hardware 300 based on the spike neuron model.

The neuromorphic hardware 300 may include a plurality of neuron devices310. Each neuron device 310 may be electrically connected to an externalsignal generating section and one or more other neuron devices 310, andreceive an input signal that changes over time. Each neuron device 310may output a spiked output signal to the one or more other neurondevices 310, according to the input pattern of a plurality of inputsignals. Such neuromorphic hardware 300 may be configured as a liquidstate machine.

The reservoir computing system 200 according to the present embodimentmay use such neuromorphic hardware 300 as the reservoir 130.Specifically, the plurality of the input nodes 10 in the input layer 120are each connected to a corresponding neuron device 310. Furthermore,one or more output nodes 22 are connected to a portion of the neurondevices 310 among the plurality of neuron devices 310. For example, oneoutput node 22 receives a spiking train from one neuron device 310, asshown in FIG. 6.

Here, the one output node 22 may be operable to output, as the outputvalue, a value (T₁-T₀, T₂-T₁, T_(d)-T_(d−1)) representing the spikeinterval of the output signal output by the reservoir 130. The outputnode 22 may supply the vector generator section 220 with suchinter-spike intervals. The vector generator section 220 may generate thed-dimensional delay coordinate vector by performing the same operationas used for a digital signal on the inter-spike intervals (T₁-T₀, T₂-T₁,. . . T_(d)-T_(d−1)). In this way, the reservoir computing system 200according to the present embodiment can use a spike neural network orthe like as the reservoir 130.

The reservoir computing system 200 described above can performinglearning, testing, and the like as long as it is possible to acquire anoutput signal from a portion of the nodes among the plurality of nodesin the reservoir 130. Accordingly, the reservoir 130 does not need toform all of the nodes as physical nodes. In this case, the reservoir 130may be a device having a fine structure or the like. The reservoir 130may be a device forming a spin system, a propagation system for surfaceacoustic waves, a microwave conducting system, or the like. Furthermore,the reservoir 130 may be a device that includes a ferromagneticmaterial, a phase change material, or the like.

The reservoir 130 may use an input/output response of such a device.Specifically, the reservoir 130 may be a physical reservoir that outputsan electrical, magnetic, optical, mechanical, thermal, or acousticoutput signal in response to an electrical, magnetic, optical,mechanical, thermal, or acoustic input signal. The physical reservoirmay include a metal layer, a ferroelectric layer, a ferromagnetic layer,a phase change material layer, and/or the like formed on a substrate.

The physical reservoir may receive an input signal from one or moreinput sections and propagate the input signal therein. By propagatingthe input signal therein in a plurality of directions, the physicalreservoir may change the signal components in a complex manner andoperate as a plurality of virtual nodes. The physical reservoir mayoutput an output signal from an output section according to the inputsignal being propagated in one or more output sections or according tothe effect of the input signal. Even if the reservoir computing system200 is such a physical reservoir, there is no need to exchange signalswith virtual nodes, and therefore the physical reservoir can be used asthe reservoir 130 by using signal input sections and output sectionsalong with the signals.

Here, if output signals are received from m output sections of thereservoir 130, for example, m output nodes 22 may respectively receivecorresponding output signals. The vector generator section 220 maygenerate d₁, d₂, . . . , d_(m)-dimensional delay coordinate vectors foreach of the m output signals x₁(t), x₂(t), . . . , x_(m)(t), forexample, as shown in the following expression. As shown in Expression 5,the reservoir computing system 200 may generate d₁+d₂+ . . .+d_(m)-dimensional delay coordinate vectors.

$\begin{matrix}{\mspace{79mu} {\left. {x_{1}(t)}\rightarrow\left\{ {{x_{1}(t)},{x_{1}\left( {t - T} \right)},\ldots \mspace{14mu},{x_{1}\left( {t - {\left( {d_{1} - 1} \right)T}} \right)}} \right\} \right.\mspace{79mu} \left. {x_{2}(t)}\rightarrow\left\{ {{x_{2}(t)},{x_{2}\left( {t - T} \right)},\ldots \mspace{14mu},{x_{2}\left( {t - {\left( {d_{2} - 1} \right)T}} \right)}} \right\} \right.\mspace{79mu} \vdots \mspace{79mu} \left. {x_{m}(t)}\rightarrow\left. \left\{ {{x_{m}(t)},{x_{m}\left( {t - T} \right)},\ldots \mspace{14mu},{x_{m}\left( {t - {\left( {d_{m} - 1} \right)T}} \right)}} \right\} \rightarrow\left( {{x_{1}(t)},\ldots \mspace{14mu},{x_{1}\left( {t - {\left( {d_{1} - 1} \right)T}} \right)},{x_{2}(t)},\ldots \mspace{14mu},{x_{2}\left( {t - {\left( {d_{2} - 1} \right)T}} \right)},\ldots \mspace{14mu},,\ldots \mspace{14mu},\left. \quad{{x_{m}(t)},\ldots \mspace{14mu},{x_{m}\left( {t - {\left( {d_{m} - 1} \right)T}} \right)}} \right)} \right. \right. \right.}} & {{Expression}\mspace{14mu} 5}\end{matrix}$

The reservoir computing system 200 according to the present exampledescribed above may include a plurality of output nodes, in order toaccount for connection failure of the output nodes. Such a reservoircomputing system 200 is described using FIG. 7. FIG. 7 shows amodification of the reservoir computing system 200 according to thepresent embodiment. In the reservoir computing system 200 according tothe present modification, components having substantially the sameoperation as components of the reservoir computing system 200 shown inFIG. 2 are given the same reference numerals and descriptions thereofare omitted.

The reservoir computing system 200 according to the present modificationfurther includes a first output node 24, a second output node 26, afirst vector generator section 222, and a second vector generatorsection 224. The first output node 24 may be connected to a first nodeamong the plurality of nodes of the reservoir 130. The second outputnode 26 may be connected to a second node among the plurality of nodesof the reservoir 130.

The first vector generator section 222 may be operable to generate thedelay coordinate vector, according to the output value from the firstoutput node 24. The second vector generator section 224 may be operableto generate the delay coordinate vector according to the output valuefrom the second output node 26. The adaptive filter 230 may be operableto weight the delay coordinate vector received from the first vectorgenerator section 222 or the second vector generator section 224, andoutput the result as the output data. In this case, the learningprocessing section 240 may update different weights for each path as thelearning results.

Here, the adaptive filter 230 may be operable to output the output databased on a plurality of output values received by the second output node26, in response to the path leading to the adaptive filter 230 from thereservoir 130 via the first output node 24 failing. In other words, thereservoir computing system 200 of the present includes a plurality ofpaths that are each capable of learning independently, and may beoperable to perform the learning, testing, or the like using paths thatare undamaged among the plurality of paths. In this way, if thereservoir computing system 200 is actually implemented as a hardwaredevice, the reservoir computing system 200 can operate by using otherpaths when a wiring failure or the like occurs in one of the paths, andcan improve the lifespan of the system.

Furthermore, if a plurality of paths are included, the reservoircomputing system 200 may be operable to perform the learning, testing,and the like using the plurality of paths. In this case, the outputlayer 210 may include two or more output nodes connected respectively totwo or more nodes among the plurality of nodes of the reservoir 130.

Furthermore, the reservoir computing system 200 may include a pluralityof vector generator sections corresponding to the plurality of outputnodes. Instead, the reservoir computing system 200 may include onevector generator section, and this one vector generator section maygenerate a plurality of delay coordinate vectors correspondingrespectively to the plurality of output nodes. One or more vectorgenerator sections may generate d₁, d₂, . . . , d_(m)-dimensional delaycoordinate vectors as shown by Expression 5.

The adaptive filter 230 may be operable to output the output data basedon results obtained by weighting the plurality of output values outputfrom two or more output nodes at a plurality of timings using aplurality of weights. In this way, by using a plurality of output nodes,it is possible to reduce the number of pieces of time-series dataacquired from one output node and to enhance the learning performance.Furthermore, if a failure occurs in one path while performing thelearning and testing using a plurality of paths from a plurality ofoutput nodes, the reservoir computing system 200 may continue thelearning and testing by using paths excluding this one path.

In the manner described above, the reservoir computing system 100 andthe reservoir computing system 200 according to the present embodimentmay be operable to realize a physical system having nonlinear dynamicsas the reservoir 130. For example, a semiconductor laser has a nonlinearoscillation characteristic or has a nonlinear characteristic that isfurther strengthened according to outside disturbances or the like, andcan therefore be used as the reservoir 130. The following describes anexample of a reservoir 130 using a semiconductor laser.

FIG. 8 shows a first exemplary configuration of the reservoir 130according to the present embodiment. FIG. 8 shows an example in whichthe reservoir 130 is a semiconductor laser formed on a substrate. Thereservoir 130 includes a substrate 410, a semiconductor laser 420, awaveguide 430, a mirror 440, a photodetection section 450, and an ADconverting section 460.

The substrate 410 may be a semiconductor substrate. The substrate 410 ispreferably a substrate operable to form a semiconductor laser. Forexample, the substrate 410 is a silicon substrate. The substrate 410 maybe a compound substrate. The semiconductor laser 420 may be operable tooutput laser light in response to having current injected thereto. Thesemiconductor laser 420 may be operable to be formed on the substrate410. The semiconductor laser 420 is a DFB (Distribution Feedback) laser,for example.

The waveguide 430 is formed on the substrate 410, and may be operable topropagate laser light that is output by the semiconductor laser 420 tothe photodetection section 450. The waveguide 430 may be a siliconwaveguide formed in a silicon substrate. The mirror 440 is provided inthe waveguide 430 and may be operable to reflect a position of the laserlight output by the semiconductor laser 420. The mirror 440 may beoperable to function as a half-mirror that reflects a portion of thelaser light toward this semiconductor laser 420 and transparently passesthe remaining portion in a manner to propagate to the photodetectionsection 450. The mirror 440 may be a Bragg reflective mirror formed in asilicon waveguide. In this case, the mirror 440 may be operable to havethe reflectivity thereof adjusted by having current injected thereto.

The photodetection section 450 may be operable to receive the laserlight output from the waveguide 430. The photodetection section 450 maybe operable to output an electrical signal corresponding to theintensity of the received laser light. The photodetection section 450may be operable to output an electrical signal corresponding tofluctuation over time of the laser light output. The photodetectionsection 450 may be a photodiode or the like.

The AD converting section 460 may be operable to convert the electricalsignal output by the photodetection section 450 into a digital signal.The AD converting section 460 may convert the electrical signal inputthereto into a digital signal at substantially constant predeterminedtime intervals, according to a clock signal or the like. The ADconverting section 460 may include an AD converter of a type such assequential comparison, flash, pipeline, or digital. For example, the ADconverting section 460 may include circuit elements of a MOS structureformed on a silicon substrate or the like.

The semiconductor laser 420 and the waveguide 430 described above may beformed integrally on the substrate 410. Furthermore, the semiconductorlaser 420, the waveguide 430, and the mirror 440 are preferably formedintegrally on the substrate 410. For example, the semiconductor laser420, the waveguide 430, and the mirror 440 are formed by applyingmachining such as etching, deposition, or the like to a siliconsubstrate. The photodetection section 450 may also be formed integrallyon the substrate 410. Furthermore, the AD converting section 460 mayalso be formed integrally on the substrate 410.

The reservoir 130 of the first exemplary configuration described abovemay output a response that is nonlinear with respect to the time axis,using the semiconductor laser 420 that has a feedback function due tothe mirror 440. Specifically, the semiconductor laser 420 may beoperable to perform laser oscillation according to a current injectedthereto, while receiving external disturbances due to feedback lightcaused by the mirror 440. Due to such external feedback, thesemiconductor laser 420 preferably causes the intensity of the outputlight to have a time waveform that vibrates self-excitedly, even whenthe position and reflectivity of the mirror 440 are substantiallyconstant.

Furthermore, the reservoir 130 according to the first exemplaryconfiguration may be operable to output a more complex time response bymodulating the position and/or reflectivity of the mirror 440. Forexample, if a plate-shaped mirror 440 is provided, the reservoir 130 mayperform modulation by moving the position of this mirror 440 with anactuator or the like. Furthermore, if a Bragg reflection mirror isprovided as the mirror 440, the reservoir 130 may modulate the currentinjected into the Bragg reflection mirror. Alternatively, the reservoir130 may modulate the current injected into the semiconductor laser 420.

In this way, the reservoir 130 of the first exemplary configuration canmodulate the light output of the laser light input to the photodetectionsection 450 in a temporally complex manner, and can therefore be adoptedin a reservoir computing system 100. For example, the reservoir 130supplies each of the output nodes 20 corresponding to the output layer140 shown in FIG. 1 with time-series digital signals S₁ to S_(n) outputby the photodetection section 450 at timings from t₁ to t_(n). In thisway, the reservoir 130 of the first exemplary configuration can operateas the reservoir 130 of the reservoir computing system 100 shown in FIG.1.

Furthermore, as an example, the reservoir 130 of the first exemplaryconfiguration supplies the output node 22 of the output layer 210 shownin FIG. 2 with a time-series digital signal sequence (S₁, S₂, . . . ,S_(n)) output by the photodetection section 450 at the timings from t₁to t_(n). In this way, the reservoir 130 of the first exemplaryconfiguration can operate as the reservoir 130 of the reservoircomputing system 200 shown in FIG. 2.

The reservoir 130 of the first exemplary configuration described abovecan lengthen the delay time for inputting the external feedback light asthe distance D between the semiconductor laser 420 and the mirror 440becomes longer, and can therefore output a more complex time response.However, if the semiconductor laser 420 and the mirror 440 are formedintegrally on the substrate 410 that is made of silicon or the like, forexample, it is difficult to perform manufacturing stably by separatingthe semiconductor laser 420 and the mirror 440 by a distance D ofapproximately several centimeters. In other words, it is possible toachieve miniaturization by forming these components integrally in thereservoir 130, but the distance D is more than 1 cm and it is difficultto make the delay time greater than or equal to approximately 10 ps.

Furthermore, the semiconductor laser 420 and the mirror 440 can beformed independently and further separated by a distance D, but thereservoir 130 increases in size by this separation distance.Furthermore, if the semiconductor laser 420 and the mirror 440 areformed independently, a precise adjustment of the optical axis isnecessary, and this increases the manufacturing cost. The followingdescribes a reservoir 130 that enables lengthening of the distance Dwithout performing a precise adjustment of the optical axis.

FIG. 9 shows a second exemplary configuration of the reservoir 130according to the present embodiment. FIG. 9 shows an example in whichthe reservoir 130 is a laser apparatus using optical fiber. In thereservoir 130 of the second exemplary configuration, components havingsubstantially the same operation as components of the reservoir 130 ofthe first exemplary configuration shown in FIG. 8 are given the samereference numerals, and descriptions thereof are omitted. The reservoir130 includes a semiconductor laser 420, a photodetection section 450, anAD converting section 460, optical fiber 470, an optical circulator 480,and an optical coupler 490.

The semiconductor laser 420 of the second exemplary configuration may beoperable to output laser light to the optical fiber 470. Thesemiconductor laser 420 may be a device with a fiber pigtail attachedthereto. The optical fiber 470 is provided between each device and maybe operable to propagate the laser light. For example, the optical fiber470 is provided between the semiconductor laser 420 and the opticalcirculator 480. Furthermore, the optical fiber 470 is provided betweenthe optical circulator 480 and the optical coupler 490.

The optical circulator 480 may be a three-port type of opticalcirculator that includes a first port 482, a second port 484, and athird port 486. As an example, the optical circulator 480 outputs thelight input to the first port 482 from the second port 484 and outputsthe light input to the second port 484 from the third port 486. FIG. 9shows an example in which the second port 484 of the optical circulator480 is connected to the semiconductor laser 420, the light input to thefirst port 482 is supplied to the semiconductor laser 420, and the laserlight output from the semiconductor laser 420 is output from the thirdport 486.

The optical coupler 490 may be a one-input two-output optical couplerthat includes a first input 492, a second output 494, and a third output496. The optical coupler 490 may cause the light input thereto to besplit or combined. FIG. 9 shows an example in which the light input fromthe first input 492 by the optical coupler 490 is split to the secondoutput 494 and the third output 496. The optical coupler 490 may outputthe laser light from the second output 494 to the photodetection section450. Furthermore, the reservoir 130 may supply the semiconductor laser420 with the laser light output from the third output 496 of the opticalcoupler 490 as the external feedback light.

The reservoir 130 of the second exemplary configuration described abovemay have the optical coupler 490 output a portion of the laser lightoutput from the semiconductor laser 420 to the outside and feed theremaining portion of the laser light back to the semiconductor laser420. The reservoir 130 of the second exemplary configuration may havethe optical fiber 470 provided between the semiconductor laser 420 andthe optical circulator 480 and between the optical circulator 480 andthe optical coupler 490. Specifically, the delay time for inputting theexternal feedback light to the semiconductor laser 420 can be easilyadjusted according to the length of the optical fiber 470.

For example, it is possible to set the length of the optical fiber 470to be from approximately 10 cm to approximately several kilometers. Inthis way, the reservoir 130 of the second exemplary configuration canrealize a longer delay time at a lower cost compared to the reservoir130 of the first exemplary configuration. Due to such external feedbacklight, the reservoir 130 can output a response that is nonlinear withrespect to the time axis. Specifically, the semiconductor laser 420 cancause the intensity of the output light to have a time waveform thatvibrates self-excitedly due to the external feedback light, even whenthe splitting ratio of the optical coupler 490 is substantiallyconstant.

In this way, the reservoir 130 of the second exemplary configuration cancause the light output of the laser light input to the photodetectionsection 450 to fluctuate in a temporally complex manner, and cantherefore be adopted in a reservoir computing system 100, in the samemanner as the reservoir 130 of the first exemplary configuration.However, since the devices such as the semiconductor laser 420, thephotodetection section 450, the AD converting section 460, the opticalcirculator 480, and the optical coupler 490 are provided independently,the reservoir 130 of the second exemplary configuration has a largersize than the reservoir 130 of the first exemplary configuration.

Therefore, in the present embodiment, a reservoir 130 is described thatcan reduce the cost and further lengthen the distance D, withoutincreasing the size. Such a reservoir 130 may include a laser apparatus500 such as shown in FIG. 10.

FIG. 10 shows an exemplary configuration of the laser apparatus 500according to the present embodiment. The laser apparatus 500 furtherlengthens the delay time of the external feedback light to the laseroscillation source by causing the laser light to propagate in a ringresonator. The laser apparatus 500 includes a substrate 510, a laser520, a feedback waveguide 530, an optical splitter 540, a first ringresonator 550, and a second ring resonator 560.

The substrate 510 may be a semiconductor substrate. The substrate 510 ispreferably a substrate operable to form a semiconductor laser or thelike. The substrate 510 may be a silicon-based substrate. The substrate510 may be one of a silicon substrate, a compound semiconductorsubstrate, a glass substrate, a ceramic substrate, and the like. FIG. 10shows an exemplary configuration in the XY plane that is substantiallyparallel to the front surface of the substrate 510. The substrate 510may have optical elements and the like formed thereon by havingmachining such as deposition, burying, etching, or the like appliedthereto in the Z direction, which is substantially perpendicular to thefront surface.

The laser 520 may be operable to output laser light in response tocurrent injected thereto. The laser 520 may be a semiconductor laserthat is operable to be formed on the substrate 510. For example, thelaser 520 is a DFB (Distribution Feedback) laser. In this case, thelaser 520 may be operable to be formed with a length from substantially100 μm to substantially 1 mm in a first direction on the substrate 510.The laser 520 is preferably formed with a length of approximatelyseveral hundred micrometers in the first direction. Furthermore, thelaser 520 may be operable to be formed with a length from substantially10 μm to substantially tens of micrometers in a second direction that isorthogonal to the first direction on the substrate 510. The laser 520 ispreferably formed with a length of tens of micrometers in the seconddirection. The example in FIG. 10 uses the X direction as the firstdirection and the Y direction as the second direction.

The feedback waveguide 530 may be operable to feed the light output fromthe laser 520 back to the laser 520. The feedback waveguide 530 may beoperable to propagate the laser light output from one end of the laser520 to the other end of the laser 520. For example, the feedbackwaveguide 530 propagates the light output from one edge of the laser 520to another edge. The feedback waveguide 530 may be operable to be formedon the substrate 510. If the substrate 510 is a silicon substrate, thefeedback waveguide 530 may be a silicon waveguide path formed on thissilicon substrate.

The feedback waveguide 530 may have an elliptical or ovular shape on thefront surface of the substrate 510. The feedback waveguide 530 may havea loop shape with a length in the second direction, which isperpendicular to the first direction, that is less than the length inthe first direction. In this case, the feedback waveguide 530 mayinclude a first linear portion 532 and a second linear portion 534 as apart of this loop shape. The first linear portion 532 may be a portionof the loop shape that extends in the first direction. The second linearportion 534 may be a portion of the loop shape of the feedback waveguide530 that extends in the first direction on the opposite side from thefirst linear portion 532.

FIG. 10 shows an example in which the laser 520 is arranged in thesecond linear portion 534 of the feedback waveguide 530. The loop lengthof the feedback waveguide 530 may be operable to be formed to be fromhundreds of micrometers to several centimeters or more. The loop lengthof the feedback waveguide 530 is preferably formed to be approximatelyseveral millimeters.

The optical splitter 540 is provided in the path of the feedbackwaveguide 530, and may be operable to output a portion of the lightpropagated by the feedback waveguide 530 to the outside of the substrate510. FIG. 10 shows an example in which the optical splitter 540 isarranged in the second linear portion 534 of the feedback waveguide 530.The optical splitter 540 may be a one-input two-output optical couplerthat has a first input 542, a second output 544, and a third output 546.The optical splitter 540 may split the light input from the first input542 to the second output 544 and the third output 546. FIG. 10 shows anexample in which the optical splitter 540 outputs a portion of the lightinput from the first input 542 to the outside from the second output544, and outputs the remaining portion of the light from the thirdoutput 546 to the first linear portion 532.

The first ring resonator 550 may be operable to optically connect to thefeedback waveguide 530. The first ring resonator 550 is opticallyconnected to the feedback waveguide 530, and may be operable to input aportion of the laser light output from the laser 520 from the opticalconnection portion, and to cause this light to circulate within thefirst ring resonator 550. Furthermore, the first ring resonator 550 mayreturn a portion of the laser light that has been circulated in theresonator to the feedback waveguide 530. The first ring resonator 550may return the laser light to the feedback waveguide 530 one portion ata time, every time the laser light makes a circulation. In this way, thefirst ring resonator 550 can function as a delay element that delays thetime according to the distance that the laser light has circulated.

The first ring resonator 550 may be formed with a loop shape on thefront surface of the substrate 510. For example, the first ringresonator 550 may have a shape that is circular, elliptical, ovular, orthe like. The loop length of the first ring resonator 550 may be lessthan the loop length of the feedback waveguide 530. The first ringresonator 550 preferably has a diameter that is greater than or equal toapproximately 10 μm and less than or equal to hundreds of micrometers.The first ring resonator 550 more preferably has a diameter that isgreater than or equal to approximately 10 μm and less than or equal to100 μm. The first ring resonator 550 may be formed of substantially thesame material as the feedback waveguide 530.

The first ring resonator 550 may be arranged in the first linear portion532 of the feedback waveguide 530, for example. In this case, theoptical splitter 540 may be arranged in the feedback waveguide 530between the output of the laser 520 and the optical connection portionconnecting to the first ring resonator 550. The first ring resonator 550may include an electrode 552.

The electrode 552 may be operable to control the first ring resonator550. For example, the electrode 552 changes the refractive index n ofthe silicon crystal by injecting current thereto. Accordingly, if thesubstrate 510 is a silicon substrate, the electrode 552 is formed on aportion of the light waveguide of the first ring resonator 550, and theelectrode 552 can change the velocity v=c/n of the light beingpropagated in the first ring resonator 550 by injecting the current.Specifically, if the laser apparatus 500 includes the electrode 552, theelectrode 552 can be made operable to adjust the wavelength of the lightbeing circulated in the first ring resonator 550 by having currentapplied to the electrode 552 from the outside.

One or more of the second ring resonators 560 may be provided. Each ofthe one or more second ring resonators 560 may be operable to opticallyconnect to the first ring resonator 550 either directly or indirectlyvia another of the second ring resonators 560. FIG. 10 shows an examplein which a plurality of the second ring resonators 560 are provided. Theplurality of second ring resonators 560 may be optically connected inseries.

FIG. 10 shows an example in which the second ring resonator 560 that ispositioned at a first end, among the plurality of second ring resonators560, is optically connected to the first ring resonator 550. Theplurality of second ring resonators 560 may be arranged in a firstdirection, which is a direction away from the first ring resonator 550,from the second ring resonator 560 at the first end. In other words, theplurality of second ring resonators 560 may be arranged along the firstlinear portion 532.

Each second ring resonator 560 may be formed with a loop shape on thefront surface of the substrate 510. Each second ring resonator 560 mayhave a shape that is circular, elliptical, ovular, or the like, forexample. The loop length of each second ring resonator 560 may be lessthan the loop length of the feedback waveguide 530. Each second ringresonator 560 preferably has a diameter that is greater than or equal toapproximately 10 μm and less than or equal to hundreds of micrometers.Each second ring resonator 560 more preferably has a diameter that isgreater than or equal to approximately 10 μm and less than or equal to100 μm. Each second ring resonator 560 preferably has substantially thesame shape as the first ring resonator 550. Each second ring resonator560 may be formed of substantially the same material as the feedbackwaveguide 530. Some or all of the second ring resonators 560 may includean electrode 562.

The electrode 562 may be operable to control the resonance of thecorresponding second ring resonator 560, in the same manner as theelectrode 552 of the first ring resonator 550. Specifically, if thelaser apparatus 500 includes one or more electrodes 562, the laserapparatus 500 can be made operable to adjust the wavelength of the lightbeing circulated in the second ring resonator 560 in which the electrode562 is formed, by supplying a current to the electrode 562 from theoutside. The laser apparatus 500 may include such an electrode in atleast one of the first ring resonator 550 and the one or more secondring resonators 560. If the laser apparatus 500 is used as the reservoir130, the input signal may be input to such an electrode. Specifically,the input node 10 of the input layer 110 may be operable to supply theelectrode with the input signal corresponding to the input data.

The second ring resonator 560 that is optically connected to the firstring resonator 550 may circulate a portion of the laser light that iscirculated in the first ring resonator 550. The second ring resonator560 may supply a portion of the laser light being circulated to anadjacent second ring resonator 560 connected thereto in series.Furthermore, the second ring resonator 560 may return a portion of theremaining laser light being circulated to the first ring resonator 550.The second ring resonator 560 optically connected to the first ringresonator 550 may supply a portion of the laser light being circulatedto each of the adjacent second ring resonator 560 and the first ringresonator 550, every time the laser light makes a circulation.

In the same manner, each second ring resonator 560 may sequentiallycirculate the laser light, in order of the serial connection from thefirst ring resonator 550. Furthermore, each second ring resonator 560may supply each of the two adjacent second ring resonators 560 with aportion of the laser light being circulated. In this way, the laserlight input to the first ring resonator 550 from the feedback waveguide530 passes through each of a plurality of paths that cause onecirculation and a plurality of circulations in the first ring resonator550 and a plurality of paths that cause various circulations in theplurality of second ring resonators 560, and again returns to thefeedback waveguide 530.

In other words, the laser apparatus 500 can supply the laser 520 withlaser light having various delay times, as the external feedback light.Furthermore, the laser apparatus 500 can include, in the externalfeedback light, laser light that has made a plurality of circulations inthe first ring resonator 550 and the one or more second ring resonators560. Accordingly, the laser apparatus 500 can supply the laser 520 withlaser light that has been delayed by a time exceeding 1 ns, for example,as the external feedback light.

Here, the laser 520, the feedback waveguide 530, the optical splitter540, and the first ring resonator 550 may be operable to be formed onthe same substrate 510. Furthermore, the second ring resonators 560 maybe operable to be formed on the substrate 510. In other words, the laserapparatus 500 may be formed integrally with the silicon substrate. Inthis way, the laser apparatus 500 can have a length in the firstdirection that is less than or equal to approximately 1 mm and a lengthin the second direction of approximately hundreds of micrometers, whichis smaller than the length in the first direction, for example.

As an example, a case is considered in which the laser apparatus 500 hasthe laser 520 with a length of approximately 500 μm in the firstdirection and the feedback waveguide 530 with a loop length ofapproximately 1 mm formed on the substrate 510. In this case as well, byproviding a total of approximately ten resonators including the firstring resonator 550 and the plurality of second ring resonators 560, thelaser apparatus 500 can generate external feedback light having a delaytime of approximately 1 ns, which is equivalent to a resonator length ofsubstantially 30 cm. The diameter of each of the first ring resonator550 and the plurality of second ring resonators 560 may be approximately40 μm.

In the manner described above, the laser apparatus 500 according to thepresent embodiment can realize a feedback path that is 100 times or morelonger when compared to the size of the apparatus, with a small size andat low cost. Accordingly, the laser apparatus 500 can function as thereservoir 130 by supplying the photodetection section 450 with theoutput laser light and converting the result into a digital signal usingthe AD converting section 460, in the same manner as described in FIG. 8and FIG. 9.

FIG. 11 shows an exemplary configuration of a cross section of the laserapparatus 500 shown in FIG. 10 across the line A-B. The substrate 510may include a silicon substrate 512 and a silicon oxide film 514.Specifically, FIG. 11 shows an example in which the laser 520, thefeedback waveguide 530, the optical splitter 540, the first ringresonator 550, and the second ring resonators 560 are formed on thesilicon oxide film 514 of the substrate 510.

The laser 520 may be a semiconductor laser formed by a group III-Vcompound semiconductor. The laser 520 may include a first electrode 522and a second electrode 524. The laser 520 can output laser light from anactive region, by having current injected between the first electrode522 and the second electrode 524. Furthermore, the feedback waveguide530, the optical splitter 540, the first ring resonator 550, and thesecond ring resonators 560 may be made of silicon crystal or the likeand formed on the silicon oxide film 514 of the substrate 510. Thefollowing uses FIG. 12 and FIG. 13 to describe the light output of sucha laser apparatus 500.

FIG. 12 shows an example of the light output characteristic of a laserapparatus serving as a comparison target. The laser apparatus serving asa comparison target is a laser apparatus with a configuration obtainedby removing the first ring resonator 550 and the second ring resonators560 from the laser apparatus 500 shown in FIG. 10 and FIG. 11.Specifically, the laser apparatus serving as the comparison target is anexample of a laser apparatus that has a feedback loop of approximately 1mm. In FIG. 12, the horizontal axis indicates time and the vertical axisindicates the relative value of the optical power. It should be notedthat FIG. 12 shows an example of calculation results obtained through asimulation.

The laser apparatus serving as the comparison target begins laseroscillation when current is injected and an inverse distribution isformed. The laser apparatus serving as the comparison target beginsnonlinear laser oscillation in which the optical power vibrates.However, since the delay of the external feedback light is short in thelaser apparatus serving as the comparison target, the amplitude of thevibration of the optical power eventually becomes small when this delaytime has passed, and the laser apparatus serving as the comparisontarget outputs a substantially constant optical power.

FIG. 13 shows an example of the light output characteristic of the laserapparatus 500 according to the present embodiment. The laser apparatus500 may have substantially the same configuration as the laser apparatus500 shown in FIG. 10 and FIG. 11. In FIG. 13, the horizontal axisindicates time and the vertical axis indicates the relative value of theoptical power. It should be noted that FIG. 13 shows an example ofcalculation results obtained through a simulation, in the same manner asin FIG. 12.

The laser apparatus 500 begins laser oscillation when current isinjected and an inverse distribution is formed. The laser apparatus 500begins nonlinear laser oscillation in which the optical power vibrates.It is understood that since the external feedback light includes a delaytime exceeding 1 ns, the laser apparatus 500 continues the vibration ofthe optical power and a nonlinear input/output response is maintained.In the manner described above, the laser apparatus 500 according to thepresent embodiment can continue the nonlinear input/output response, andcan therefore be used as the reservoir 130 of the reservoir computingsystem 100 and the reservoir computing system 200 according to thepresent embodiment.

The laser apparatus 500 according to the present embodiment describedabove is an example in which the second ring resonators 560 are arrangedin series in the first direction. Here, among the plurality of secondring resonators 560 arranged in series, the second ring resonator 560 atone end may be optically connected to the first ring resonator 550, andthe second ring resonator 560 at the other end may be opticallyconnected to the first linear portion 532 of the feedback waveguide 530.Furthermore, a portion of the plurality of second ring resonators 560may branch. The laser apparatus 500 may generate a longer time delay byforming the plurality of second ring resonators 560 with more complexpaths.

The laser apparatus 500 according to the present embodiment is describedas an example including the first ring resonator 550 and the second ringresonators 560, but the present invention is not limited to this. Thelaser apparatus 500 may include only the first ring resonator 550.

The laser apparatus 500 may further include an optical amplifier, lightswitching element, or the like between the first ring resonator 550 andthe laser 520 in the feedback waveguide 530. The optical amplifier, thelight switching element, or the like may be operable to operateaccording to a control signal from the outside. If the delay time of theexternal feedback light returning from the first ring resonator 550 isless than 100 μs, for example, the optical amplifier, the lightswitching element, or the like may be controlled to be in the OFF statesuch that this external feedback light does not reach the laser 520.

Furthermore, if the delay time of the external feedback light returningfrom the first ring resonator 550 is greater than or equal to 100 μs,for example, the optical amplifier, the light switching element, or thelike may be controlled to be in the ON state such that this externalfeedback light reaches the laser 520. In this way, the laser apparatus500 can supply the laser 520 with the external feedback light having adelay time that is greater than or equal to a predetermined delay time.

FIG. 14 shows an exemplary hardware configuration of a computeraccording to the embodiment of the invention. A program that isinstalled in the computer 800 can cause the computer 800 to function asor perform operations associated with apparatuses of the embodiments ofthe present invention or one or more sections (including modules,components, elements, etc.) thereof, and/or cause the computer 800 toperform processes of the embodiments of the present invention or stepsthereof. Such a program may be executed by the CPU 800-12 to cause thecomputer 800 to perform certain operations associated with some or allof the blocks of flowcharts and block diagrams described herein.

The computer 800 according to the present embodiment includes a CPU800-12, a RAM 800-14, a graphics controller 800-16, and a display device800-18, which are mutually connected by a host controller 800-10. Thecomputer 800 also includes input/output units such as a communicationinterface 800-22, a hard disk drive 800-24, a DVD-ROM drive 800-26 andan IC card drive, which are connected to the host controller 800-10 viaan input/output controller 800-20. The computer also includes legacyinput/output units such as a ROM 800-30 and a keyboard 800-42, which areconnected to the input/output controller 800-20 through an input/outputchip 800-40.

The CPU 800-12 operates according to programs stored in the ROM 800-30and the RAM 800-14, thereby controlling each unit. The graphicscontroller 800-16 obtains image data generated by the CPU 800-12 on aframe buffer or the like provided in the RAM 800-14 or in itself, andcauses the image data to be displayed on the display device 800-18.

The communication interface 800-22 communicates with other electronicdevices via a network 800-50. The hard disk drive 800-24 stores programsand data used by the CPU 800-12 within the computer 800. The DVD-ROMdrive 800-26 reads the programs or the data from the DVD-ROM 800-01, andprovides the hard disk drive 800-24 with the programs or the data viathe RAM 800-14. The IC card drive reads programs and data from an ICcard, and/or writes programs and data into the IC card.

The ROM 800-30 stores therein a boot program or the like executed by thecomputer 800 at the time of activation, and/or a program depending onthe hardware of the computer 800. The input/output chip 800-40 may alsoconnect various input/output units via a parallel port, a serial port, akeyboard port, a mouse port, and the like to the input/output controller800-20.

A program is provided by computer readable media such as the DVD-ROM800-01 or the IC card. The program is read from the computer readablemedia, installed into the hard disk drive 800-24, RAM 800-14, or ROM800-30, which are also examples of computer readable media, and executedby the CPU 800-12. The information processing described in theseprograms is read into the computer 800, resulting in cooperation betweena program and the above-mentioned various types of hardware resources.An apparatus or method may be constituted by realizing the operation orprocessing of information in accordance with the usage of the computer800.

For example, when communication is performed between the computer 800and an external device, the CPU 800-12 may execute a communicationprogram loaded onto the RAM 800-14 to instruct communication processingto the communication interface 800-22, based on the processing describedin the communication program. The communication interface 800-22, undercontrol of the CPU 800-12, reads transmission data stored on atransmission buffering region provided in a recording medium such as theRAM 800-14, the hard disk drive 800-24, the DVD-ROM 800-01, or the ICcard, and transmits the read transmission data to network 800-50 orwrites reception data received from network 800-50 to a receptionbuffering region or the like provided on the recording medium.

In addition, the CPU 800-12 may cause all or a necessary portion of afile or a database to be read into the RAM 800-14, the file or thedatabase having been stored in an external recording medium such as thehard disk drive 800-24, the DVD-ROM drive 800-26 (DVD-ROM 800-01), theIC card, etc., and perform various types of processing on the data onthe RAM 800-14. The CPU 800-12 may then write back the processed data tothe external recording medium.

Various types of information, such as various types of programs, data,tables, and databases, may be stored in the recording medium to undergoinformation processing. The CPU 800-12 may perform various types ofprocessing on the data read from the RAM 800-14, which includes varioustypes of operations, processing of information, condition judging,conditional branch, unconditional branch, search/replace of information,etc., as described throughout this disclosure and designated by aninstruction sequence of programs, and writes the result back to the RAM800-14. In addition, the CPU 800-12 may search for information in afile, a database, etc., in the recording medium. For example, when aplurality of entries, each having an attribute value of a firstattribute is associated with an attribute value of a second attribute,are stored in the recording medium, the CPU 800-12 may search for anentry matching the condition whose attribute value of the firstattribute is designated, from among the plurality of entries, and readsthe attribute value of the second attribute stored in the entry, therebyobtaining the attribute value of the second attribute associated withthe first attribute satisfying the predetermined condition.

The above-explained program or software modules may be stored in thecomputer readable media on or near the computer 800. In addition, arecording medium such as a hard disk or a RAM provided in a serversystem connected to a dedicated communication network or the Internetcan be used as the computer readable media, thereby providing theprogram to the computer 800 via the network.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Smalltalk, C++ or the like, andconventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the userscomputer, partly on the users computer, as a stand-alone softwarepackage, partly on the users computer and partly on a remote computer orentirely on the remote computer or server. In the latter scenario, theremote computer may be connected to the users computer through any typeof network, including a local area network (LAN) or a wide area network(WAN), or the connection may be made to an external computer (forexample, through the Internet using an Internet Service Provider). Insome embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to individualize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

While the embodiments of the present invention have been described, thetechnical scope of the invention is not limited to the above describedembodiments. It is apparent to persons skilled in the art that variousalterations and improvements can be added to the above-describedembodiments. It is also apparent from the scope of the claims that theembodiments added with such alterations or improvements can be includedin the technical scope of the invention.

The operations, procedures, steps, and stages of each process performedby an apparatus, system, program, and method shown in the claims,embodiments, or diagrams can be performed in any order as long as theorder is not indicated by “prior to,” “before,” or the like and as longas the output from a previous process is not used in a later process.Even if the process flow is described using phrases such as “first” or“next” in the claims, embodiments, or diagrams, it does not necessarilymean that the process must be performed in this order.

As made clear from the above, the embodiments of the present inventioncan realize a nonlinear input/output characteristic while achieving asmall size and low cost, and can be adopted as a reservoir when areservoir computing system is implemented as actual hardware.

What is claimed is:
 1. A reservoir computing system comprising: areservoir including a laser apparatus, wherein the laser apparatusincludes a laser, a feedback waveguide that is operable to feed lightoutput from the laser back to the laser, an optical splitter that isprovided in a path of the feedback waveguide and is operable to output aportion of light propagated in the feedback waveguide to outside thefeedback waveguide, and a first ring resonator that is operable to beoptically connected to the feedback waveguide; an input node operable tosupply the reservoir with an input signal corresponding to input data;an output node operable to output an output value corresponding to lightoutput by the reservoir in response to the input data; and an outputsection operable to output output data corresponding to the outputvalue.
 2. The reservoir computing system according to claim 1, whereinthe laser apparatus further includes one or more second ring resonators,and each of the one or more second ring resonators is operable to beoptically connected to the first ring resonator directly or indirectlyvia another of the second ring resonators.
 3. The reservoir computingsystem according to claim 2, wherein a plurality of the second ringresonators are operable to be optically connected in series, and thesecond ring resonator positioned at a first end among the plurality ofsecond ring resonators is operable to be optically connected to thefirst ring resonator.
 4. The reservoir computing system according toclaim 2, wherein at least one of the first ring resonator and the one ormore second ring resonators includes an electrode for controllingresonation.
 5. The reservoir computing system according to claim 4,wherein the input node is operable to supply the electrode with an inputsignal corresponding to the input data.
 6. The reservoir computingsystem according to claim 1, wherein the output section includes anadaptive filter that is operable to output output data based on resultsobtained by weighting a plurality of the output values output from theoutput node at a plurality of timings with a plurality of weights. 7.The reservoir computing system according to claim 6, wherein theplurality of weights are operable to be set such that the reservoircomputing system outputs the output data that is expected for the inputdata.
 8. The reservoir computing system according to claim 7, furthercomprising: a learning processing section that is operable to update theplurality of weights in a manner to reduce an error between output dataoutput by the adaptive filter in response to the input node beingsupplied with training input data and expected output data that isexpected for the training input data.
 9. The reservoir computing systemaccording to claim 1, wherein the laser apparatus further comprises: oneor more second ring resonators, wherein each of the one or more secondring resonators is operable to be optically connected to the first ringresonator directly or indirectly via another of the second ringresonators.
 10. The reservoir computing system according to claim 9,wherein the laser apparatus further comprises a plurality of the secondring resonators operable to be optically connected in series, and thesecond ring resonator positioned at a first end among the plurality ofsecond ring resonators is operable to be optically connected to thefirst ring resonator.
 11. The reservoir computing system according toclaim 10, wherein the feedback waveguide includes a first linear portionthat extends in a first direction, and the plurality of second ringresonators are operable to be arranged along the first linear portion.12. The reservoir computing system according to claim 11, wherein thefeedback waveguide has a loop shape in which a length thereof in asecond direction, which is perpendicular to the first direction, is lessthan a length thereof in the first direction.
 13. The reservoircomputing system according to claim 11, wherein the laser is operable tobe arranged in a second linear portion that extends in the firstdirection on a side opposite the first linear portion in the feedbackwaveguide.
 14. The reservoir computing system according to claim 9,wherein at least one of the first ring resonator and the one or moresecond ring resonators includes an electrode for controlling resonation.15. The reservoir computing system according to claim 9, wherein a looplength of each of the first ring resonator and the one or more secondring resonators is less than a loop length of the feedback waveguide.16. The reservoir computing system according to claim 1, wherein theoptical splitter is operable to be arranged between an output of thelaser and an optical connection portion with the first ring resonator inthe feedback waveguide.
 17. A reservoir computing system comprising: areservoir including a laser apparatus, wherein the laser apparatusincludes a laser, a feedback waveguide that is operable to feed lightoutput from the laser back to the laser, an optical splitter that isprovided in a path of the feedback waveguide, a first ring resonatorthat is operable to be optically connected to the feedback waveguide,and one or more second ring resonators; an input node operable to supplythe reservoir with an input signal corresponding to input data; anoutput node operable to output electrical signal output datacorresponding to light output by the reservoir in response to the inputdata; and an output section operable to convert the electrical signaloutput data to a digital signal value.
 18. The reservoir computingsystem according to claim 17, wherein the laser, the feedback waveguide,the optical splitter, and the first ring resonator are operable to beformed on a substrate.
 19. The reservoir computing system according toclaim 18, wherein the substrate is a silicon-based substrate.
 20. Thereservoir computing system according to claim 17, wherein each of theone or more second ring resonators is operable to be optically connectedto the first ring resonator directly or indirectly via another of thesecond ring resonators.